[FFmpeg-devel] [PATCH] aac: Clip large (legal) PNS/IS values

[Robert Swain]

2009/10/10 Alex Converse <alex.converse at gmail.com>:
> On Fri, Oct 9, 2009 at 7:34 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> On Fri, Oct 09, 2009 at 12:38:17PM -0400, Alex Converse wrote:
>>> On Fri, Oct 9, 2009 at 9:37 AM, Robert Swain <robert.swain at gmail.com> wrote:
>>> > 2009/10/6 Alex Converse <alex.converse at gmail.com>:
>>> >> Large intensity stereo and PNS indices are legal. Clip them instead of
>>> >> erroring out. A magnitude of 100 corresponds to 2^25 so the will most
>>> >> likely result in clipped output anyway.
>>> >>
>>> >> None of the conformance streams fall in the range that need to be clipped.
>>> >
>>> > Looks OK to me as long as it isn't arbitrary clipping but rather
>>> > something needed/allowed within and specified by the specification.
>>> >
>>>
>>> The clipping is semi-arbitrary. It was designed such that after the
>>> third patch the ranges are the same as regular scalefactors. However
>>> these large vales (even when clipped) should cause cause output
>>> clipping on any streams that aren't purely academic exercises.
>>
>> IMHO, either
>> A. it can be in a valid stream -> we should support it
>> B. it can not be in a valid stream -> error concealment (silence /repeat
>> ? last block / something better) should be applied
>> cliping seems always wrong (to me) unless mandated by a spec.
>> switching between A and B, in the sense of "it could be valid but most
>> likely is not, lets ask the user" can be done by using
>> AVCodecContext.error_recognition
>>
>
> It's a mix between that (the error recognition case) and the fact that
> even if you decode it properly there isn't going to be a difference
> between clipping and not.
>
> For instance let's look at intensity stereo. If the IS scalefactor is
> 100, associated then the gain is 2^-25 The only way for the channel
> receiving the intensity stereo to get something that doesn't round to
> zero is for the source channel to be clipped beyond belief.
>
> If that gain of 2**-25 seems reasonable to you, then consider another
> legal IS scalefactor, 512. 512 corresponds to a gain of 2**128. That
> is larger than the maximum finite float.
>
> IS scalefactors are coded differentially from 0 with a maximum step of
> +/-60 which is another reason why very large values are unlikely.

As long as the scalefactor values are valid within the specification,
I think we should only clip them if it is provable that doing so does
not alter the output of any valid stream, academic exercise or not.

Best regards,
Rob